A variety of gas-barrier resins have heretofore been used. Among them, polyvinylidene chlorides, polyacrylonitriles and ethylene/vinyl alcohol copolymers have been particularly known as gas-barrier resins. However, use of the polyvinylidene chlorides and polyacrylonitriles has been refrained from the environmental point of view while the ethylene/vinyl alcohol copolymers are much dependent for their gas-barrier property on the humidity permitting the gas-barrier property to decrease under highly humid conditions.
A film obtained by depositing an inorganic material on the surface of a base material has also been known to impart gas-barrier property to the packing material. However, the film is very expensive. Besides, the film on which the inorganic material is deposited has poor flexibility and poor adhesive property to the base material or to other resin layers.
In order to solve the above problem, there have been proposed a gas-barrier film obtained by forming, on the base material, a film comprising a water-soluble high molecular material A, a water-soluble or water-dispersing high molecular material B and an inorganic lamellar compound (patent document 1), a gas-barrier film obtained by applying a layer that contains a metal compound onto the surface of a layer comprising a mixture of a poly(meth)acrylic acid-type polymer and polyalcohols (patent document 2), and a gas-barrier coating material (patent document 3) containing a polyvinyl alcohol, an ethylene/maleic acid copolymer, and a divalent or more highly valent metal compound.
There have further been proposed a gas-barrier resin composition obtained by reacting a polycarboxylic acid-type polymer, a crosslinking agent having 2 to 4 functional groups that react with the carboxyl group, and divalent or more highly valent metal ions, so that the polycarboxylic acid-type polymer forms the crosslinked portions due to the crosslinking agent and the crosslinked portions due to the divalent or more highly valent metal ions, and that the weight ratio of the polycarboxylic acid-type polymer and the crosslinking agent is from 99.9/0.1 to 65/35 (patent document 4), and a gas-barrier film obtained by forming a gas-barrier layer on at least one surface of a thermoplastic resin film, the gas-barrier layer being formed of a polyacrylic acid crosslinked with a crosslinking agent containing an epoxy compound that has not less then 3 epoxy groups per a molecule, the crosslinking agent being contained in an amount of 1 to 100 parts by mass per 100 parts by mass of the polyacrylic acid (patent document 5).
Patent document 1: JP-A-9-151264
Patent document 2: JP-A-2000-931
Patent document 3: JP-A-2004-115776
Patent document 4: JP-A-2003-171419
Patent document 5: JP-A-2002-210207